Removing sulfate from Minnesota wastewater
For Minnesota utilities and businesses, finding effective ways to remove sulfate from municipal or industrial wastewater has become an urgent priority, in part due to recent judicial rulings and legislative changes. Biological treatment, in which sulfate is removed from water biologically by bacteria, holds great potential as a cost-effective solution for a growing number of Minnesota facilities.
Let’s take a closer look at what’s happening with biological sulfate treatment in Minnesota and what it means for your wastewater projects.
Uncertainty drives action
Minnesota legislation, which limits sulfate concentrations in discharges to wild rice waters to 10 mg/L, has been the most stringent sulfate standard in the country for decades. Now additional work is underway to update this standard; however, with previous rulings such as a judge’s dismissal of a Minnesota Pollution Control Agency proposal, the outcome remains uncertain.
This uncertainty increases the urgency for utilities and businesses to develop a portfolio of viable sulfate-removal technologies to meet a range of potential permit limits. Active or passive biological treatment can be a cost-effective sulfate-treatment option, either as a standalone technology or as pretreatment.
Figure 1: The sulfur cycle as relevant to biological sulfate treatment in water. This figure shows how sulfur changes between sulfate, sulfide, and sulfur, and related reactions that affect biological sulfate removal technologies.
Established technologies for sulfate removal
The most established technologies for achieving sulfate concentrations below 250 mg/L are membrane separation (also called reverse osmosis) and ion exchange. Both technologies produce a brine waste that can be expensive and energy-intensive to manage. A water-quality project Barr completed for the Minnesota Office of Management and Budget suggests that implementation of membrane separation to treat sulfate could increase sewer rates by an estimated $500–$1,000 per household annually.
Biological sulfate removal has been applied primarily to treat high-sulfate waters in mining applications. High initial concentrations of sulfate mean smaller reactors are needed to remove a given mass of sulfate. While biological treatment is unlikely to achieve 10 mg/L, it could be used to meet higher permit limits with lower costs and energy. This type of technology could provide a more cost-effective alternative, but it requires completion of current and future applied research and demonstration studies to determine effectiveness in low-sulfate applications.
Factors that impact biological sulfate treatment effectiveness
The effectiveness of biological sulfate treatment depends on many factors, including reactor configuration, temperature, carbon source, and nutrient supply. It’s also limited by biochemistry. Biological sulfate treatment relies on microbes that breathe with sulfate instead of oxygen; however, breathing with sulfate is much less efficient than breathing with oxygen. As a result, biological sulfate treatment systems require larger volumes and retention times than more traditional aerobic biological treatment.
Another limitation of biological sulfate treatment is the final fate of sulfate, which should be retained as a stable byproduct. The default for these types of metabolisms is to release sulfur as hydrogen sulfide gas, but systems can be engineered to retain the sulfur within iron-sulfide minerals. As the country’s leader in iron-ore production, Minnesota is well positioned to implement iron-bearing byproducts from mining for this application. This makes implementation of sulfate treatment in Minnesota—especially in northern Minnesota—more viable than in other areas, as treatment material would not need to be transported as far.
Current state of biological sulfate treatment development
Collaboration with university researchers is needed to refine promising technologies.
Bench-scale and demonstration-scale research is underway at the University of Minnesota to develop technologies for full-scale trials. To make progress in this area, industry stakeholders and consultants will need to collaborate with university researchers to evaluate and refine promising technologies.
As regulations and research evolve, we expect biological sulfate removal to become increasingly viable as a treatment alternative for Minnesota industries and utilities. While the timeline to implementation depends on study results, risk tolerance, and future legislative outcomes, potentially viable applications may be ready within three to 10 years.
To learn more about hosting a demonstration study at your site or how this technology relates to your operations, contact Barr.
About the author
Ali Ling, former environmental engineer and water/wastewater process engineer at Barr, specialized in connecting basic science to engineering outcomes for clients in various industries, using tools like bench and pilot testing and process modeling. Ali helped facilitate Barr’s involvement in applied research and university collaborations. She has an academic background in microbial ecology and applied microbiology.